Interpreting the Quantum
World

Einstein's Last Stand

Einstein moved to the USA after
the rise of nazism. He continued to be dissatisfied with
the formulation of quantum mechanics and tried to find ways
to challenge the theories. Seemingly unable to use the uncertainty
principle for this purpose, he developed the EPR paradox
with two younger colleagues at Princeton, Boris Podolsky
and Nathan Rosen. Their argument centered around a quantum
system consisting of a pair of particles (A and B) which
are created with opposite spin, i.e.: the overall system
has no net spin.

The particles move apart at the speed of light and when
they are widely separated, the spin of one of the particles,
say A, is measured and found to point 'up'. The classical
interpretation is simple: particle 'B' was always in a
spin 'down' state. The quantum mechanical interpretation
is more tricky as the Copenhagen Interpretation says that
the spin of particle 'A' has no definite value until it
is measured. Or, to quote Bohr: '… the state of
an atomic system before a measurement is not defined but
only has the possibility of certain values with associated
probabilities'. At this point, it must produce an instantaneous
effect on particle 'B' to give the opposite spin. This
requires 'action at a distance' or faster than light communication!
Faster than light action was completely disallowed by
Einstein's theory of special relativity. Einstein and
his colleagues said that there must be hidden information
which is not contained in the wave function of the system.
Einstein debated this at length with Bohr until his death
in 1955.